Three-piece metal cans frequently are fabricated by forming a flat metal blank, usually rectangular in shape, into a tubular configuration with the lateral ends or edges being lapped and welded together as a longitudinal seam. End closures are then secured across the open ends of the tubular configuration to complete the can formation. The term "tubular" is not restricted to a circular cross-section, as square or other shaped cans can be fabricated with this same approach. Continuously welded seams can also be used for fabricating structures other than cans.
To make a seam weld in electric-resistent can welders, opposed roller electrodes are continuously tracked along the lapped blank edges, one on the inside and one on the outside of the tubular configuration, as the blank is moved between the rotating electrodes and a large welding current pulsed between the opposed electrodes is conducted through the lapped edges at the small contact regions between the electrodes, heating the edges to a mastic state as a series of closely adjacent "spot welds" for producing a smooth weld seam.
The steel blanks typically have a thin protective surface coating, such as of tin; but as such coating has relatively poor conductivity and in order to minimize the needed welding currents, the blank edges to be lapped are generally not coated. Nonetheless, welding heat can splatter the coating; and splatter accumulations on the roller electrode surface increase electrical resistance and the needed welding current, and reduce the weld quality. To counter this, a copper wire electrode aligned along each exposed lapped blank edge is fitted in a circumferential groove on the adjacent roller electrode, and the wire electrodes are squeezed under a significant compressive force between the opposed roller electrodes and respective blank edges.
The outer roller electrode is rotated under power, sychronized with the can blank conveyor, and effectively rotates the inner roller electrode and advancing blank edges and wire electrodes as the seam is being welded. Increased resistance against free inner electrode rotation and/or increased welding current and temperatures, can cause localized heating or straining of the wire electrode and can break it. Should the wire electrode break, the operation of the can welder must be immediately halted, or direct shorting and/or arcing between the roller electrodes can burn and operatively destroy them.
Modern automatic can welders operate at very high amperage and power levels, and at linear welding speeds in excess of 70 meters per minute for yielding production rates in excess of 600 cans per minute. Equipment and material suppliers suggest higher welding power levels and output rates will soon be possible or required, for example by improved electronically produced power wave forms and frequencies. In fact, actual and proposed shifts to economical can blank materials suggest the need of higher power levels on existing can welders to form suitable welds.
One continuing development effort relates to improving inner roller electrodes, which must carry the electrical power between gapped moving parts, such as between a stationary stator supported by the welder frame and a rotor rotatably carried on the stator and shaped to roll along the lapped blank edges. Roller bearings between the gapped stator and rotor components allow for the relative rotation, and insulators encasing the bearings electrically isolate the gapped stator and rotor components from one another. Electrically conductive means, most commonly in the form of an electrically conductive liquid but also in the form of special mechanically cooperating contacts, are contained between the gapped stator and rotor components to provide a specific intended path for the required welding amperage.
Conventionally, the electrically conductive means has been via a conductive liquid sealed between the stator and rotor components, and mercury has been extensively used as it withstands the high electrical power levels while remaining a liquid. However, its conductivity is only about 2% that of the conventional cooper alloy roller electrodes, and thus represents a disproportionately large part of the overall electrical resistance against the welding current. Moreover, as mercury is highly toxic, including its vapor which can be released even at room temperatures, whereby current workplace regulations, environmental restrictions, and the potential of consequential liability now are challenging its use.
My prior U.S. Pat. No. 4,780,589 issued on Oct. 25, 1988; U.S. Pat. No. 4,940,873 issued on Jul. 10, 1990; and U.S. Pat. No. 5,089,862 issued on Feb. 18, 1992, each related to ROLLER ELECTRODES FOR ELECTRIC-RESISTANCE WELDING MACHINE that totally eliminated mercury and its consequential liability as the electrically conductive liquid, and further offered improved weld output and quality.
My improved roller electrodes use a substantially non harmful conductive liquid comprised as a gallium dominant eutectic mixture, by weight approximating 61% gallium (Ga), 25% indium (In), 13% tin (Sn), and 1% zinc (Zn), which has electrical and thermal conductivities almost four times better than mercury. The electrode surfaces exposed to the conductive liquid or intense welding currents further are coated with a very thin layer of a platinum family material, preferably rhodium. A control limits the temperature of the coolant circulated through the roller electrodes to above approximately 10 degrees C.
One cause of roller electrode failures and/or wire electrode breakages is the increased drag against rotor rotation exhibited in most failed electrodes. Mercury knowningly contributes to the increased drag, as when it is exposed to the copper alloy electrode components, it converts from a liquid to an amalgam that begins pasty but eventually becomes rock hard. With respect to this phenomenon, for improving shelf life it has long been recommended to store a filled but unused mercury roller electrode at a chilled temperature and to move it regularly. For this reason and also in an effort to eliminate the liability of handling a mercury product, electrode suppliers have recently tried to ship the roller electrode empty and require the user to fill it with mercury when use is to begin. Even my roller electrodes exhibit conductive liquid breakdown, except only to becoming pasty and not rock hard like the mercury amalgam.
Increased rotor drag is also caused by failed roller bearings. Specifically, each roller bearing generally is comprised of inner and outer raceways separated by many small balls constrained therebetween, with some circumferential spacing between the balls. The balls thus can roll along and between the raceways, overall moving circumferentially in the same direction as the rotor rotation. Close tolerances and smoothness of the component surfaces, and good lubrication are required for free ball movement. Most failed bearings suggest operation while hot and/or dirty, with roughened surfaces and burnt lubricant; so corrective design efforts have attempted to improve bearing quality, seals, cooling and/or lubrication.
Notwithstanding these known potential causes for failure and grand efforts to eliminate them, and despite generally overall improved roller electrode operation, the inconsistency and lack of predictability of roller electrodes operation have always frustrated design efforts for improving these devices.
However, the inventor now has determined that a major cause of roller electrode failure relates to ineffective bearings between the stator and rotor. In analyzing the problem, the inventor specifically notes that the bearings operate very close to the welding regions, within a few centimeters, and thus are within the high energy pulsed electrical fields used in the can welder. Further, the bearing raceways and balls are typically formed of a very hard bearing steel, which is both electrically and magnetically conductive.
One aspect of the problem is that although many balls are in each bearing, only those few balls instanteously located closely adjacent and on opposite sides of the force line through the bearing center and only on the loaded side of the bearing actually support the bearing load. Meanwhile, the other balls carry little or even none of the load and thus frequently can be shifted circumferentially until restrained by a cage or by the adjacent ball. Larger roller electrodes, possibly 65 mm OD or larger, offer sufficient space to use bearings having ball restraining cages; while smaller roller electrodes with less available space frequently use cageless bearings. The pulsed electrical circuit at the welding region generates a high energy magnetic field, and because the bearings can be very proximate to the welding region, the steel bearing components can be polarized. When the freely moveable and polarized balls are alternately attracted and/or repelled relative to one another, high impact collisions locally concentrated at the ball/ball or ball/cage interfaces will occur, which are believed to be a main but unappreciated cause of spald spots, or roughness observed in failed bearings.
Another aspect of the problem is traced to the cup-shaped anodized aluminum insulators commonly used to electrically isolate the gapped stator and rotor components from one another, which knowing have only a very thin anodized layer typically less than 0.002 mm thick. The inventor has determined that the assembly step of press-fitting each insulator between the rotor and the outer bearing raceway, to keep them from shifting relative to one another during operation, frequently scratches the thin anodized layer sufficiently to reduce or break down the effective insulation. This short circuit path via the steel bearings between the stator and rotor components, and the limited ball/race interfaces, can generate arcing and/or high current densities and component temperatures; each leading to overheating and burnt lubricant, and the generation of spald spots.
While inner roller electrodes are known to be problemsome, so too are the forming rolls (commonly known as hour glass rolls) used to shape the originally flat blank into the cylindrical can shape, which also are very small with very small bearings and operate in the same high energy pulsed electrical fields of the can welder. In fact, two such rolls have bearings located generally as close to the welding region as the inner roller electrode bearings. These rolls occasionally have exhibited increased drag against rotation, precluding proper blank movement past the roller electrodes. While roll bearings are not subjected to electrical power, their steel construction and operation in the high energy fields of the can welder can lead to magnetic polarization, poor operation and ultimate failure.